Study On Process And Joint Corrosion Behavior Of Aluminum/Steel Dissimilar Metals Friction Stir Welding

Energy conservation and emission reduction is the consensus of all countries in the world,and structural lightweight is the key to achieve energy conservation and emission reduction.The aluminum alloy,due to the advantages of high specific strength and specific rigidity,corrosion resistance and good plasticity,was used to replace the steel in the field of the carrier tool manufacturing.The friction stir welding?FSW?is one of the most promising welding methods for aluminum/steel dissimilar metals.Firstly,the finite element model of Q235 steel and 6061 aluminum alloy?T6?flat butt joint with a thickness of 2.5mm was established by using the finite element software ANSYS,and the temperature field and residual stress field in the FSW joint of aluminum/steel dissimilar metal were calculated by using the model.The evolution rule of the process parameters to temperature field and residual stress field were explored.The small hole method was used to measure the longitudinal residual stress of the joint to verified the heat source model.Secondly,based on the finite element calculation results,the flat butt welding of the 6061 aluminum alloy and Q235 steel with the thickness of 2.5mm was carried on by the positive offset?the steel side of the mixing needle?with FSW equipment.The microstructure of the joint under the different welding processes was observed by using the OM,SEM,XRD and EDS,and the intermetallic compounds?IMCs?in the interface transition zone?ITZ?were characterized.The mechanical properties of the joint were tested by electronic universal testing machine and microhardness tester,and the evolution rule of the microstructure and mechanical properties of the FSW joint of aluminum/steel dissimilar metal was explored.Finally,the corrosion behaviors of FSW joint of aluminum/steel dissimilar metal under marine environment were tested by electrochemical workstation.The morphology of the joint after corrosion was observed by SEM to verify the corrosion mechanism of the ITZ.The results show that:?1?The temperature field and residual stress field of aluminum/steel dissimilar metals FSW joint was asymmetrical,which the peak temperature and peak longitudinal residual stress??_x?appears at the partial steel side of the weld nugget zone?WNZ?.With the increasing of rotational speed,the peak temperature increases and the peak?_x increases.With the increasing of welding speed,the peak temperature and the peak?_x increase.The peak temperature increases slightly with the increase of pin offset and?_x decreases slightly.The simulation results matched reasonably well with experiment results.?2?With the increasing of welding speed,the burr and flash on the surface of the joint decrease first and then form groove,the banded structure shortens,the density decreases,the size of the ITZ decreases,the microhardness of the WNZ decreases first and then increases,the microhardness of the TMAZ and HAZ increases,and the tensile strength of the joint increases first and then decreases.With the increasing of welding speed,the microhardness of HAZ increases first and then decreases.At the same time,the burr and flash on the weld surface increase obviously,the banded structure becomes longer,the density increases,the size of the ITZ increases,the microhardness of the WNZ increases first and then decreases,the hardness of the TMAZ and HAZ decreases,and the tensile strength of the joint decreases gradually.?3?The corrosion behaviors of the FSW joint of aluminum/steel dissimilar metal mainly occur in the ITZ of the joint.IMCs and Fe-rich phase particles in the ITZ accelerates the corrosion behaviors of the joint.The corrosion mechanism of the FSW joint of aluminum/steel dissimilar metal is mainly electrochemical corrosion due to potential difference between the IMCs and Q235 steel,6061 aluminum alloy,The corrosion resistance sequence of the IMCs is as follows:Al Fe₃>Al Fe>Fe₂Al₅.Secondly,there is potential difference between the Fe-rich phase particles and the surrounding Al elements,which leads to electrochemical corrosion.